1. Field of the Invention
The present invention relates to enhancing the stability of thickening agent solutions, including but not limited to guar solutions. More specifically, the present invention relates to enhancing the stability of thickening agent solutions by sterilizing these solutions with UV light, and/or by adding a surfactant to these solutions.
2. Background Information
Natural and synthetic polymers containing hydroxy groups have been used as thickeners for foods, coatings, paints, explosive slurries, oil well fluids, cosmetics and other personal care products, and many other functional applications.
One class of polymers that have been widely used as suspending and viscosity agents are polygalactomannans. Polygalactomannans are polysaccharides composed principally of galactose and mannose units and are usually found in the endosperm of leguminous seeds such as guar, locust bean, honey locust, flame tree, tara, fenugreek and the like. Guar flour, for example, is composed mostly of a galactomannan which is essentially a straight chain mannan with single membered galactose branches. The mannose units are linked in a 1-4-.beta.-glycosidic linkage and the galactose branching takes place by means of a 1-6 linkage on mannose units in an irregular manner. The ratio of galactose to mannose in the guar polymer is about one to two.
Locust bean gum is also a polygalactomannan gum of similar molecular structure in which the ratio of galactose to mannose is one to four. Guar and locust bean gum are the preferred sources of the polygalactomannans, principally because of the commercial availability thereof.
Polygalactomannan may be used in either its natural state (i.e., pure guar gum or locust bean gum) or may be derivatized. Derivatized polygalactomannans include one or more non-ionic and/or ionic groups. Examples of such polygalactomannans include hydroxypropyl guar, hydroxyethyl guar, carboxymethyl guar, carboxymethyl hydroxypropyl guar and the like having varying degrees of substitution and molar substitution. Such derivatized polygalactomannans are sold by Rhone-Poulenc Inc. under the trade names Jaguar 8000 (hydroxypropyl guar), Jaguar 8710 (carboxymethyl guar) and Jaguar 8600 (carboxymethyl hydroxypropyl guar). Many commercially available starting guar materials may contain small amounts of additives such as borax, glyoxal and the like. These starting materials are expressly covered as constituting part of the present invention.
The term "degree of substitution" as employed herein is the average substitution of functional groups per anhydro sugar unit in the polygalactomannan gums. In guar gum, the basic unit of the polymer consists of two mannose units with a glycosidic linkage and a galactose unit attached to a hydroxyl group of one of the mannose units. On the average, each of the anhydro sugar units contains three available hydroxyl sites. A degree of substitution of three would mean that all of the available hydroxyl sites have been esterified with functional groups. A particularly preferred functional group is the carboxymethyl group, with good results obtained with starting materials having a degree of substitution of between about 0.0 and about 3.0, specifically including materials having a degree of substitution ranging from about 0.10 to about 0.15.
Similarly, the term "molar substitution" as employed herein is the average number of moles of functional groups per anhydro sugar unit in the polygalactomannan gum. A particularly preferred functional group is the hydroxypropyl group, with good results obtained with starting materials having a molar substitution of between about 0.0 and about 3.0. In a preferred embodiment, the resulting polysaccharide is carboxymethyl hydroxypropyl guar having a molar substitution of hydroxypropyl groups of between about 0.25 and about 0.35 and a degree of substitution of carboxymethyl groups of between about 0.10 and about 0.15.
While the use of polygalactomannans, and guar gum in particular, as thickening agents has been met with great success, it is still desired to improve the physical properties of the guar gum when dispersed in a solution such as water. One such property is guar's ability to retain its viscosity over extended periods. Solutions of guar in water are known to lose viscosity over time.
As will be known to those skilled in the art, the guar endosperm is commonly referred to as "purified splits", "double purified splits" or "triple purified splits" depending upon the degree of purification. "Purified splits" are obtained by mechanical separation of the endosperm from the hull and germ of the guar seed in as pure and intact a form as possible with no other processing steps. Repeating the process produces double purified splits. Repeating the process again produces triple purified splits. Splits are then ground into guar powder.
There are several methods to sterilize solutions including, for example, steam sterilization, pasteurization, and chemical sterilization. Steam sterilization of guar solutions results in polymer degradation and loss of viscosity of the guar solution; this reduction in viscosity can be as high as 80%. Pasteurization can be similarly ineffective in that certain spore forming bacteria such as Bacillus species, which have been found in guar, usually cannot be killed by simple pasteurization processes; pasteurization can even facilitate spore germination. Chemical sterilization such as the use of biocides and bactericides as well as chemicals such as ethylene oxide also have drawbacks. Use of various biocides and/or preservatives is often restricted or even prohibited with certain products such as food products and personal care products. Use of ethylene oxide can lead to formation of trace amounts of ethylene glycol and 2-chloroethanol on guar.
Wellington, Soc. Pet. Eng. J., 23:901-912 (1983) and Mitchell, et al., Food Hydrocolloids, 5:141-143 (1991) report the use of antioxidants to stabilize various galactomannans such as xanthan gum and guar gum. Neither of these references report the use of UV light sterilization or the use of surfactants, alone or in combination, to increase the stability of thickening agent solutions.
The use of ultraviolet (UV) radiation for sterilization of various products is known. Many industrial systems are available for UV sterilization, especially for sterilization of water. For example, laboratory water purification systems such as the Milli-Q UV Plus Water System, commercially available from the Milli-Pore Corporation, uses UV light to sterilize water. The use of UV light to sterilize guar solutions or other thickening agents, however, has not been reported.
Thus, improved methods for sterilizing or otherwise treating guar solutions so they retain their viscosity stability over time are needed.
The entire disclosure of U.S. provisional application Ser. No. 60/040,182, filed Mar. 12, 1997, is considered as being part of this disclosure and is hereby incorporated by reference herein.